1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus, such as a printer or a copier, which transfers a toner image onto a record medium such as paper by an electrophotographic method.
2. Description of Related Art
Generally, in an image forming apparatus that forms a color image by an electrophotographic method, monochrome toner images of Y (yellow), M (magenta), C (cyan), and K (black) are transferred onto an intermediate transfer belt and combined into a composite image (primary transfer), and the composite image is transferred onto a record medium (secondary transfer). At the time of secondary transfer, a transfer bias having a reverse polarity to the toner is applied to a secondary transfer roller in order to form an electric field between the intermediate transfer belt and the secondary transfer roller so that the toner can be moved from the intermediate transfer belt to the record medium. This transfer bias is preferably subjected to constant voltage control which enables favorable transfer regardless of the coverage (ratio of toner in a fixed printed area) of an image to be printed.
As for such constant voltage control, for example, Japanese Patent Laid-Open Publication No. 2008-275946 discloses that while a secondary transfer current is monitored, an applied voltage is controlled so that the secondary transfer voltage will not exceed a predetermined and stored upper limit and will not come below a predetermined and stored lower limit.
Meanwhile, it is necessary to separate the record medium from the intermediate transfer belt immediately after the secondary transfer, and for smooth separation of the record medium, an antistatic needle that removes an electric charge from the record medium is provided immediately after the transfer roller, and a DC voltage is applied to the antistatic needle. Japanese Patent Laid-Open Publication No. 2010-249872 discloses that a value obtained by subtracting a detected antistatic current from a detected secondary transfer current is taken as a transfer contribution current and that a bias voltage applied to the secondary transfer roller is controlled such that the transfer contribution current will be kept within a predetermined range.
Incidentally, for separation of the record medium from the image carrier (intermediate transfer belt) after the transfer and especially for separation of thin paper, it is effective to apply a superimposed voltage of an AC voltage and a DC voltage to the antistatic member. However, when a current flowing through the transfer roller with an AC voltage applied to the antistatic member is measured, a conventional DC measuring device, which instantaneously measures a current, performs erroneous operations because the current flowing through the transfer area during transfer fluctuates in accordance with the period of the AC voltage applied to the antistatic member. Therefore, when the antistatic member to be applied with an AC voltage is employed in an apparatus that is controlled in a manner as described in Japanese Patent Laid-Open Publication No. 2008-275946, a transfer voltage will be set higher gradually, thereby resulting in excess transfer.